A key prosecution witness says an ex-fugitive on trial in New Mexico in the slaying of an Oklahoma couple committed the killings because the husband and wife had lied to him.

Casslyn Welch, the cousin and girlfriend of defendant John McCluskey, testified Wednesday that there was no plan to kill Gary and Linda Haas when she, McCluskey and another fugitive carjacked the couple at a rest stop in eastern New Mexico.

That changed after McCluskey allegedly told Welch the couple had lied about having guns inside their travel trailer. Welch acknowledged the lie set McCluskey off.

Welch testified that the initial plan was to find a remote spot far off Interstate 40 where the Haases could be tied up and left.

Welch says she and the fugitives wanted a four- to five-hour head start.

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A key witness in the New Mexico murder trial of a prison fugitive acknowledged Wednesday that she lied to federal prosecutors about her efforts to smuggle drugs into two Arizona prisons.

Under questioning by a defense attorney, Casslyn Welch, the cousin and girlfriend of defendant John McCluskey, testified that she smuggled heroin and marijuana in dozens of color-coded balloons that she sometimes hid in a body cavity.

The account conflicted with statements she previously made to prosecutors.

Welsh, testifying for a third day, has been grilled by the defense about statements she made to police after her arrest in the August 2010 abduction and killing of Gary and Linda Haas of Tecumseh, Okla.

The slayings occurred three days after Welch said she helped McCluskey and two other inmates escape from a privately run, medium security prison near Kingman, Ariz.

One of the inmates was caught a day later in Colorado. The search for Welch, McCluskey and his former prison bunkmate Tracy Province sparked a nationwide manhunt.

McCluskey is on trial for federal carjacking and murder charges. If convicted, he could face life in prison or the death penalty.

The defense accused Welch of lying under oath and argued that her testimony was meant to satisfy prosecutors so she could receive a lighter sentence. Welch pleaded guilty last year to charges stemming from carjacking and slaying. She has yet to be sentenced but could face life in prison.

Defense attorney Gary Mitchell said Welch's attitude is "totally the opposite" in the recorded interviews compared with the measured testimony she has given this week. The interviews, he said, show her "mean-spiritedness."

Questioned about her repeated derogatory references to the victims, Welch acknowledged showing no remorse or sympathy at the time she was arrested.

Welch teared up Monday when testifying about the killings, and Mitchell brought her to tears again Tuesday when he accused her of lying to authorities about the relationship she had with McCluskey. The two often referred to each other as husband and wife, but she denied having a sexual relationship.

"You can cry and get really emotional about this, but it's a lie," Mitchell said.

Welch said the trio commandeered the Haases' truck and travel trailer after spotting the retirees at a rest stop near the Texas-New Mexico state line on Aug. 2, 2010. The goal was to "get off the grid," Welch said.

At gunpoint, the couple was forced to head west on Interstate 40 and pull off onto a lonely two-lane road. Welch said she and Province were outside the trailer when gunshots rang out. Prosecutors and testimony point to McCluskey as the triggerman.